Proppants, such as frac sand, are utilised in extractive processes using hydraulic fracturing, for example for oil and gas extraction. Hydraulic fracturing (or fracking) is a process which is used to create or extend fractures in rock formations, using the pressure of the hydraulic fluid. The fluid is typically introduced under substantial pressure via a borehole. The fractures assist in the extraction of gas, oil, water or other materials contained in a rock formation, by increasing the porosity of the rock structure.
The function of the proppant is to flow into the fractures in the rock and maintain the fractures open, so that the fractures provide increased porosity in the rock structure and allow the effective extraction of the desired material. The proppant is introduced as a slurry with the hydraulic fluid.
As such, it is important that the proppant has the appropriate physical properties. Depending upon the situation, different proppants may be used. Frac sand is a commonly used proppant material, being formed from natural sand, or sand modified to have the required characteristics. These properties include a sufficient degree of roundness, sphericity and ability to meet a required crushing parameter. One standard for frac sand is established by the American Petroleum Institute ‘Recommended Practices for Testing Sand used in Hydraulic Fracturing Operations’, RP-56, the disclosure of which is hereby incorporated by reference.
It is disclosed in U.S. patent application No 20100071902 to Zeigler to produce an artificial frac sand from naturally occurring silica sand. This discloses a process of crushing, screening and repeated pneumatic abrasion to achieve the desired sand characteristics. However, this process requires multiple passes, in part because air is not a very efficient way to transfer energy to the sand.
It is an object of the present invention to provide a process for producing frac sand which is more efficient than existing techniques.